N-Ethoxycarbonyldeacetylcolchicine and its thiocolchicine analog are highly potent agents in the P388 assay in vivo and both bind well to tubulin protein. Although highly potent in both assays were N-butyryldeacetylthiocolchicine, N-pyruvyldeacetylcolchicine and the two phenols of the thiocolchicine series, the 2- and 3-demethylthiocolchicine. Spin-labels of colchifoline, 4-hydroxymethylcolchicine and deacetylcolchicine with 2,2,5,5-tetramethyl-1-pyrrolidinyloxy-3-3carboxylic acid prepared in optically active forms have been made, but were found unsuitable for mapping the colchicine binding site on tubulin. Diacetoxydihydrofluoresceine (DADF) has been shown to be a useful reagent for the derivatization of alcohols and amines, affording on TLC-plates after exposure to ammonia and iodine vapors red materials derived from erythrosine. Two biologically potent and two not so potent colchinoids were studied by single crystal X-ray crystallography. The active and inactive colchinoids differ in their intermolecular hydrogen-bonding behavior. Tetrahydro-10-demethoxycolchicine did not bind to tubulin and was not toxic in mice, suggesting that ring C has to be aromatic and planar.